The main cause of World War II was the aggressive expansionist policies of Nazi Germany, coupled with the unresolved issues from World War I. The Treaty of Versailles, which imposed harsh penalties on Germany, fueled resentment and nationalism. Adolf Hitler’s rise to power and his aims to overturn the treaty and expand Germany’s territory led to invasions of Austria and Czechoslovakia, ultimately culminating in the invasion of Poland in 1939. This act prompted Britain and France to declare war on Germany, effectively igniting the conflict that would engulf much of the world.